1. Field of the Invention
This invention relates to a ferroresonant constant AC voltage regulator, and particularly to a ferroresonant constant AC voltage regulator for keeping always output voltage constant with respect to variation in AC input supply voltage and load current in which a saturable core is used for feedback loop.
2. Description of the Prior Art
It is important to maintain power sources in constant voltage in communication links, information processing, instrumentation and controlling devices. For this reason, various types of constant voltage regulators have heretofore been put to practical use and utilized.
FIG. 1 is a block diagram showing a ferroresonant constant AC voltage regulator which has heretofore been the most commonly used wherein a parallel resonance circuit composed of a resonance capacitor 3 and a saturable reactor 5 is connected in series to an input (commercial) power source 1 through a linear reactor 2. A load 10 is connected in parallel with said parallel resonance circuit.
Supposing that said parallel resonance circuit is resonated to a frequency of the input power source 1 at a certain voltage, electric currents Il and Ic flowing through the resonance capacitor 3 and the saturable reactor 5, respectively, are equal in amplitude and opposite in phase to each other so that their composite current becomes substantially zero, if the current higher harmonic component thereof is ignored.
When voltage of the input power source 1 varies, the composite current in said parallel resonance circuit varies so that the current passing through the linear reactor 2 and voltage drop across the same vary also, and it leads to compensating effect. Thus the voltage applied to said parallel resonance circuit, i.e., the applied voltage to the load 10 is automatically maintained at a substantially constant value.
The above described constant AC voltage regulator is a ferroresonant type device wherein saturation phenomenon of magnetic core is utilized as is well known. This type of device has widely been put to practical use as power source and the like devices, because they have many favorable characteristic properties such that parts employed are solid and number of the parts is few, besides the devices are tolerant of distortion due to thermal expansion and jamming electromagnetic field, and moreover these devices have high durability and are inexpensive.
Recently, there has been proposed such constant voltage power source device in which a linear reactor is utilized in place of using saturation phenomenon of a magnetic core as shown in FIG. 2 wherein like or corresponding parts are shown by like reference numerals throughout FIG. 1.
A series circuit composed of a linear reactor 4 and a switching circuit 7 (for example, triode AC switch, thyristor or the like) is connected in parallel to a resonance capacitor 3. An output voltage detector 9 is connected in parallel with a load 10 to create on-off control signals decided in response to output (load) voltage.
In other words, firing angle of the switching circuit 7 is controlled in response to output signals of the output voltage detector 9, whereby equivalent reactance of the linear reactor 4 is variably controlled.
More specifically, when load voltage is higher than a target value, the control therefor is made in such a manner that the firing angle is advanced to increase the current flowing through the linear reactor 4, whereby the load voltage is decreased, whilst such control is reversed in the case where the load voltage is lower than the target value.
Unlike the regulator of FIG. 1, the constant voltage power source device in FIG. 2 has no frequency dependence, little distortion of waveform, and high efficiency. For this reason, the latter device has recently increased in practical use with rapidity.
Since the circuit shown in FIG. 1 utilizes a parallel resonance circuit as mentioned above, there arise such problems that the output voltage depends upon frequency, and distortion increases or efficiency decreases with the increase of exciting current accompanied with saturation of the magnetic core and the like.
Furthermore the circuit of FIG. 2 involves such problems that operational delay of the control circuit occurs in case of, particularly, low load, and since the operating point is a sensitive point, hunting phenomena generate easily so that it is difficult to ensure stable operation.
In order to improve unstable operation in case of low load, it has been proposed to connect a dummy resistance 11 in parallel to the load 10 in addition to the circuit shown in FIG. 2 in only the case of low load as illustrated in FIG. 3.
In FIG. 3, an additional output voltage detector 9A produces a control signal under low load condition to cause an additional switching circuit 7A to conduct. As a result, the same state as that where load current increases is obtained, so that the aforementioned disadvantages in FIG. 2 may be eliminated.
Furthermore, as shown in FIG. 4, it has been also proposed to use a second linear reactor 4A in place of the dummy resistance 11.
In the constant voltage regulator of FIG. 3, since electric power is consumed by the dummy resistance 11, there are such problems that its efficiency decreases, and in addition transient variation occurs at the time of connecting and disconnecting the dummy resistance 11. While there is no problem as to electric power consumption by means of the dummy resistance 11 in the regulator of FIG. 4, there is such a problem that transient phenomenon appears at the time of connecting and disconnecting the linear reactor 4A as in the cases of FIGS. 2 and 3.
In addition to the above, since a control circuit for connecting and disconnecting the dummy resistance 11 or the second linear reactor 4A is separately provided in the cases of FIGS. 3 and 4, there are such disadvantages in that construction of the control circuit becomes complicated, and it results in increase in cost as well as decrease in reliability.